Positive displacement pump including an unloading device

ABSTRACT

A fluid transfer pump that has a housing, a body portion movable within the housing between a first position and a second position, a gear coupled to the housing, a body cavity defined partially between the body portion and the housing, and an aperture defined in the housing that selectively provides a fluid to the body cavity. When the fluid is applied to the body cavity at or above a pilot pressure, the body portion is in the first position adjacent to the gear and when the fluid is applied to the body cavity at a fluid pressure lower than the pilot pressure, the body portion is in the second position and spaced from the gear.

RELATED APPLICATIONS

This application is continuation application of U.S. application Ser.No. 16/429,341, filed Jun. 3, 2019, which is a divisional application ofU.S. application Ser. No. 15/139,428, filed Apr. 27, 2016, thedisclosures of which are hereby incorporated by reference herein.

FIELD OF THE DISCLOSURE

The present invention generally relates to a work vehicle having a fluidmanagement system, and more particularly to a pump for moving fluidthrough the fluid management system.

BACKGROUND

Agricultural equipment, such as a tractor or a self-propelledcombine-harvester, includes a prime mover which generates power toperform work. In the case of a tractor, the prime mover is a gas poweredengine or a diesel engine that generates power from a supply of fuel.The engine drives a transmission which moves wheels or treads to propelthe tractor, or other work vehicles, across ground or other surfaces. Inaddition to providing power to wheels through a transmission, tractorsoften include a power takeoff (PTO) which includes a shaft coupled tothe transmission and which is driven by the engine or a hydraulic motor.

Both gas powered and diesel powered vehicles includes a fluid managementsystem to manage the movement of a hydraulic fluid in the transmission.In different embodiments, the fluid management system manages themovement of fluid to other vehicle systems and components including adifferential, a steering system, brakes, the PTO, and various fluidreservoirs and coolers. Other systems and components which use themanaged fluid include a suspension, a hitch, a cab suspension, andselective control valves.

Fluid management systems typically include a positive displacement pumpwhich transfers the fluid between different portions of the fluidmanagement system, for instance between a differential and a hydraulicreservoir. In one known configuration, the hydraulic system and thedrivetrain share the hydraulic fluid and the drivetrain systemcirculates fluid through a fluid cooler. In one embodiment, a transferpump, typically a gear pump, moves the common fluid from thedifferential case to the hydraulic reservoir through a hydraulic filter.

Energy losses occur throughout the drivetrain when the vehicle is movingor standing still. This energy loss, also known as a parasitic loss, isa result of many factors including the movement of oil in the systemwhich exceeds the flow requirements. The excess flow is returned to areservoir or a transmission sump, and is consequently not converted toperform a function but instead results in lost energy.

Transfer pumps under different operating conditions can be a source ofthese parasitic losses, even when the vehicle's engine is idling.Attempts to reduce the parasitic losses of positive displacement pumpshave been provided by the use of additional devices such as expensiveclutches, external unloading valves, large unloading valves, pistonpumps, and variable displacement mechanisms for vane pumps. Thesesolutions, however, still develop pressure losses. Consequently, what isneeded therefore is a transfer pump that further reduces parasiticlosses without the use of costly additional devices or the additionalcomplexity associated with such devices.

SUMMARY

A transfer pump is configured to move a hydraulic fluid in a hydraulicfluid management system of a hydraulic system of a vehicle including atransmission. In one embodiment, the transfer pump moves the fluid froma differential case to a hydraulic reservoir coupled to thetransmission. The transfer pump is a positive displacement pumpincluding an unloading device, such as a plate, that is resilientlybiased in the pump during a normal operation but is moved away from thepumping gears upon the application of a pilot pressure. The result is anopen chamber defining a volume for the pump gears to turn withoutdeveloping pressure, to thereby reduce parasitic losses. In anotherembodiment, the plate is resiliently biased away from the pump duringnormal operation, but is move toward the pump upon the application of apilot pressure.

In one embodiment, there is provided a fluid transfer pump including ahousing defining a first cavity and a passage operatively coupled to asecond cavity. The passage is configured to provide fluid transfer intothe second cavity. The housing further includes an interior walldisposed in the housing and a fixed side wall disposed at an end of thehousing. A movable member is movably disposed within the housing and abias element is disposed between the wall and the movable member. Afluid applied to the passage into the second cavity changes the biasforce of the bias element and the location of the movable member withrespect to the interior wall to change a volume of the first cavity.

In another embodiment, there is provided a fluid transfer pump includinga housing defining a cavity and a passage having an inlet disposed at anexternal portion of the housing. A movable member is disposed within thecavity. The movable member defines a first chamber, wherein the volumeof the first chamber is adjustable with movement of the movable member.A gear set is disposed at the first chamber, wherein the first chamberis configured to hold a fluid. A bias element is disposed at the movablemember and is configured to adjust the location of the movable member toa first position defining a first volume of the first chamber and to asecond position defining a second volume of the first chamber.

In still another embodiment, there is provided a method of controllingthe operation of a fluid transfer pump having a housing, defining acavity, and including a bearing, located at the cavity. The methodincludes: biasing an adjustable plate disposed within the cavity at afirst location with a bias element; and applying a fluid pressure to theadjustable plate to adjust the bias of the bias element and to move theadjustable plate away from the bearing to a second location.

In a further embodiment, there is provided a fluid transfer pumpincluding a housing defining a cavity and a passage having an inletdisposed at an external portion of the housing. A movable member isdisposed within the cavity. The movable member defines a first chamber,wherein a volume of the first chamber is adjustable with movement of themovable member. A gear set is disposed at the first chamber. The gearset includes a first gear and a second gear, wherein the first chamberis defined by ends of the first gear, the second gear, and the movablemember. The first chamber is configured to hold a first volume of fluidin fluid communication with the ends of the first gear and the secondgear. A bias element is disposed at the movable member, wherein the biaselement is configured to adjust the location of the movable member to afirst position defining the first volume of the first chamber and to asecond position defining a second volume of the first chamber. Themovable member partially defines a second chamber in which a volume ofthe second chamber is adjustable with movement of the movable member.The passage is configured to direct a pilot fluid from the inlet to thesecond chamber to adjust a size of the second chamber from a first sizeto a second size, wherein, a first fluid pressure in the first chamberis greater than a second fluid pressure in the first chamber and thesecond fluid pressure is insufficient to provide a working pressure.

In another embodiment, there is provided a fluid transfer pump includinga housing defining a first cavity and a passage operatively coupled to asecond cavity. The passage is configured to provide fluid transfer intothe second cavity, wherein the housing further includes an interior walldisposed in the housing and a fixed side wall disposed at an end of thehousing. A movable member is movably disposed within the housing. A biaselement is disposed between the fixed side wall and the movable member,wherein a fluid applied to the passage into the second cavity changesthe bias force of the bias element and the location of the movablemember with respect to the interior wall to change a volume of the firstcavity. A gear is disposed at the first cavity, wherein the volume ofthe first cavity is configured to provide a fluid pressure within thefirst cavity to enable the fluid transfer pump to transfer fluid at aworking pressure. A spindle is coupled to the gear, wherein the gearpartially defines the interior wall, and the spindle extends throughboth the gear and the movable member. A fixed second sidewall, a secondsidewall bearing located at the fixed second sidewall, and movablemember bearing are located at the movable member. The movable memberdefines with the housing the first cavity and the second cavity, whereinan increase in volume of the second cavity increases the volume of thefirst cavity, and further wherein, the spindle extends from the secondsidewall bearing to the movable member bearing.

In an additional embodiment, there is provided a fluid transfer pumpincluding a housing defining a first cavity and a passage operativelycoupled to a second cavity. The passage is configured to provide fluidtransfer into the second cavity, wherein the housing further includes afixed side wall disposed at an end of the housing. A movable member ismovably disposed within the housing. A bias element is disposed betweenthe fixed side wall and the movable member, wherein a fluid applied tothe passage into the second cavity compresses the bias element andchanges the location of the movable member with respect to the fixedside wall to change a volume of the first cavity. A gear is disposed atthe first cavity and coupled to a spindle. A fixed second sidewall, asecond sidewall bearing located at the fixed second sidewall, and amovable member bearing are located at the movable member, wherein themovable member defines with the housing the first cavity and the secondcavity, wherein an increase in volume of the second cavity increases thevolume of the first cavity, and further wherein the spindle extends fromthe second sidewall bearing to the movable member bearing.

In a further embodiment, there is provided a fluid transfer pumpincluding a housing having a casing and an end cover connected to thecasing. A gear set disposed in the housing and an end plate is movablewith respect to the housing and disposed between the gear set and theend cover. The end plate is movable between a first position and secondposition, wherein in the first position the end plate defines a spacebetween the end plate and the end cover and in the second position theend plate defines a first cavity between the gear set and the end plate,wherein the end plate is sealed with the gear set to define the firstcavity. An aperture is disposed in the housing and is fluidly coupled tothe end plate, wherein pilot fluid provided at the aperture moves theend plate between the first position and the second position to providea working pressure for operation of the fluid transfer pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects of the present invention and the manner ofobtaining them will become more apparent and the invention itself willbe better understood by reference to the following description of theembodiments of the invention, taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a side perspective view of a work vehicle.

FIG. 2 is a block diagram of a fluid management system of a workvehicle.

FIG. 3 is a schematic diagram of cross-section of one embodiment of apositive displacement pump in a first condition.

FIG. 4 is a schematic diagram of a cross-section of one embodiment of apositive displacement pump in a second condition.

FIG. 5 is a schematic diagram of a cross-section of another embodimentof a positive displacement pump.

FIG. 6 is a schematic diagram of an exploded view of the embodiment ofthe positive displacement pump of FIG. 5.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thenovel invention, reference will now be made to the embodiments describedherein and illustrated in the drawings with specific language used todescribe the same. It will nevertheless be understood that no limitationof the scope of the novel invention is intended. Such alterations andfurther modifications of the illustrated apparatus, assemblies, devicesand methods, and such further applications of the principles of thenovel invention as illustrated herein, are contemplated as wouldnormally occur to one skilled in the art to which the novel inventionrelates.

The present disclosure is not exclusively directed to any type ofmachine, but rather extends to different types powered vehicles,including work vehicles such as tractors. For exemplary and illustrativepurposes, the present disclosure focuses on a utility tractor 100. InFIG. 1, for example, the tractor 100 includes a cab 102 where anoperator controls the operation of the tractor 100. The tractor 100includes an outer frame 104 to which a front and rear axle (not shown)are connected. The front axle engages a pair of front ground engagingmeans 106 (e.g., wheels) mounted thereto and the rear axle engages apair of rear ground engaging means 108 (e.g., wheels) mounted thereto.Operator controls 110, such as a steering wheel, shift lever, shiftbuttons, dashboard display, etc., are disposed in the cab 102. One ormore of these operator controls 110 is operably coupled to the machine'sdrive train, including a transmission (not shown) for controlling theoperation of the machine 100. A fluid system 200 (see FIG. 2) issupported by the frame 104 and provides a fluid, such as a hydraulicfluid, to provide for the operation of the transmission of the machine100 as well as a power take off (PTO) not shown.

As illustrated in FIG. 2, the fluid system 200 includes a transmission202 and a transmission sump 204 operatively connected to thetransmission 202. The sump 204, in one embodiment, is also coupled toprovide fluid to lubricate a front axle, a PTO, and the front brakes. Acooler (not shown) is coupled to the transmission 202, as is understoodby those skilled in the art.

A fluid reservoir 206 provides for the storage of fluid used in thefluid system 200 and is coupled to steering and brakes 208. Adifferential 210 includes a differential case 212 which is coupled tothe reservoir 206. An oil fill 214 is located at the case 212 to providefor filling of the oil of the differential case 212 as necessary. One ormore filters 216 are coupled between a port 217, of the reservoir 206,and a transfer pump 218, which is coupled to the differential case 212.An inlet 220 of the pump 218 is configured to receive filtered oil whichmoves through the filter 216. An outlet 222 is coupled to thedifferential 212. A bypass solenoid control valve 224 is coupled betweenthe port 217 and the differential case 212.

The pump 218, in one embodiment, is a low pressure gear pump configuredto transfer oil between the reservoir 206 and the differential case 212.The pump's pressure building capability is controlled byelectrohydraulic or hydraulic control of plate movement and anelectrical control signal as indicated at 226. In one embodiment, asolenoid is located at, within, or distant from the pump 218. Theelectrical control signal is provided by a controller 230, through acontrol line 231 which includes one or more processors which areconfigured to control the operation of the pump 218. The electricalcontrol signal controls the solenoid which opens or closes a hydraulicpilot signal to be applied to the pump 218.

The controller 230 is configured to execute or otherwise rely uponcomputer software applications, components, programs, objects, modules,or data structures, etc. Software routines reside in a memory, residentwithin the controller or at other external memory (not shown), or areprovided as firmware, and executed in response to the various signalsreceived and generated as described herein. The executed softwareincludes one or more specific applications, components, programs,objects, modules or sequences of instructions typically referred to as“program code”. The program code includes one or more instructionslocated in the memory, other storage devices or elsewhere, whichexecutes the control functions of the vehicle 100.

FIG. 3 and FIG. 4 are a schematic diagram of cross-section of oneembodiment of pump 218 in a first state and a second state respectively.In the illustrated embodiments, the pump 218 is a positive displacementpump which includes a housing 232 defining a cavity 233 having disposedtherein a first shaft 234 and a second shaft 236. The shaft 236 includesfirst and second ends, each of which extend externally to the housing232. The shaft is configured to be driven by a driver (not shown). Afirst gear 238 is fixed to and encircles the shaft 234 and a second gear240, which engages the first gear 238, is fixed to and encircles theshaft 236.

The first shaft 234 is supported for rotational movement within thecavity by a first bearing 242 and a second bearing 244. The second shaft236 is supported for rotational movement by a third bearing 246 and afourth bearing 248. The first and third bearings 242 and 246 aredisposed next to or at a flange 250 which forms a sidewall 251 of thehousing 232. A casing 252 forms a portion of the housing 232 and isdisposed adjacently to the sidewall 251. The bearings 242 and 246 formfluid tight seals with the shafts 234 and 236 to substantially seal thecavity 233 at the sidewall 251.

Each of the second bearing 244 and the fourth bearing 248 are disposedwithin an end plate 254, which is movable with respect to the casing 252along a direction 256, in either direction. The second and fourthbearings 244 and 248 are fluidically sealed within the end plate 254.The end plate 254, which moves in the direction 256, is fluidicallysealed with an end cover 258, such that the end plate 254 moves betweena first position of FIG. 3 to a second position of FIG. 4 and atlocations between the first and second positions. The end cover 258provides another sidewall 260 to complete the housing 232. Fluidicsealing of the end plate 254 with the end cover 258 is made by aplurality of seals 262 each of which is located at an interface of theend plate 254 and the end cover 258.

The end cover 258 defines with an aperture 264 with the casing 252. Inthe illustrated embodiment, a casing part 266 is a separable part whichis fixedly attached to the rest of the casing 252. In other embodiments,the casing 252 is single piece unitary part. The aperture 264 isfluidically coupled to a solenoid 268 which is controlled by thecontroller 230 through the line 231. The aperture 264 is furtherfluidically coupled to a cavity 269, the capacity of which varies as afunction of end plate 254 moving with respect to the gears 242 and 246.

As pilot pressure is applied by the solenoid 268, the cavity 269 expandsas the end plate 254 moves away from the gears 242 and 246 and whichforms an inside wall or an inside surface. At the same time, the cavity233, which defines a volume, expands as well as the end plate 254 movesaway from the gears 242 and 246. The capacity of the cavity 233 variesas a function of the position of the end plate 254 with respect to thebearings 238 and 240.

A first bias element 270 and a second bias element 272 are each locatedbetween the end plate 254 and the end cover 258. In differentembodiments, one or more bias elements are included. In one embodiment,the first and second bias elements 270 and 272 are helical springs, eachhaving ends engaging a groove or an aperture in the end plate 254 andthe end cover 258. Each of the bias elements 270 and 272, in arelatively uncompressed state as illustrated in FIG. 3, forces the endplate 254 away from the end cover 258 to reduce the capacity of thecavity 233. In this position, there is no pilot pressure applied throughthe aperture 264. In the absence of pilot fluid pressure being providedat the aperture 264, the force provided by the bias elements 270 and 272is sufficient to maintain the position of the end plate 254, asillustrated, which reduces the size of cavity 233 to a minimum. Inaddition, a space 274, located between the end plate 254 and the endcover 258, is at a maximum. In this condition, the pump 218 moves fluidfrom an inlet 276 to an outlet 278, both of which are fluidicallycoupled to the cavity 233.A working pressure for the operation of thegears 238 and 240 is therefore developed. With the end cover 258 in theillustrated position, the pressure developed within the cavity 233enables the shaft 234 to move the fluid from the inlet 276, through thecavity 233, through the outlet 278, and to other locations within thefluid system 100. The end clearances of each of the components arecontrolled in order to achieve an acceptable pumping efficiency.

FIG. 4 illustrates a state of the pump 218 which is provided to reducethe fluid pressure in the cavity 233 and to unload the gears 238 and 240when no fluid flow and a low energy state are desired. In thiscondition, power and/or fuel are saved to thereby reduce costs includingoperating costs and costs of repair or replacement from use.

To unload the pump 218, a pilot pressure is applied to the aperture 264through activation of the solenoid 268 by the controller 230. Uponapplication of the pilot pressure, fluid is forced into the cavity 269defined between the housing casing 252 and the end plate 254. Withsufficient pressure to overcome the bias force of the bias member 270and 272, the space 280 fills with fluid to force the end plate 254 tomove away from the gears 238 and 240. This movement reduces the size ofthe space 274.

As the space 274, of FIG. 3, is reduced in size, a space 282, which ispart of the cavity 233, expands between the gears 238 and 240 and theend plate 254 as the combined structure of the second bearing 244, thefourth bearing 248, and the end plate 254, moves towards the end cover258. As the bias elements are compressed to a shorter length, the endclearance of the gear set is increased to a point where the gears 238and 240 no longer provide sufficient working pressure for moving fluidfrom the inlet 276 to the outlet 278. When the pilot pressure isapplied, the pressure is trapped between the seals 262 near the aperture264, and this pressure is applied to the differential area of the endplate to push against the springs. Consequently, the gears run in theoil filled chamber, the cavity 233, which now has a larger capacity, andwhich lacks sufficient pressure to move fluid under pressure from theinlet 276 to the outlet 278. During this state of operation, theoperating pressure of the pump is essentially zero and the only powerrequired is the torque required to turn the gears in an oil bath.

In this embodiment, should a fault occur, the space 274 remains asdepicted in FIG. 3 and sufficient pressure is applied within the cavity233 to enable pressurized fluid flow from the inlet 276 to the outlet278. Faults can occur under different conditions, including failure ofthe solenoid 268 to operate or a decoupling of a fluid line from theaperture 264.

FIG. 5 illustrates another embodiment of a pump 300 which includes ahousing 302 having a case 304 and a cover 306 fixedly located at one endof the case 304. The case 304 includes a first cavity 308 and a secondcavity 310 separated by an inside wall 312. The first cavity 308 issubstantially closed by the cover 306, inside walls of the case 304, andone side of the wall 312, which acts a spring retainer. The wall 312includes an aperture 314 through which a ring plunger 316 is inserted.

A bias member 320 is located on a shaft 318 and is captured between aterminating end 322, which defines a retaining portion at one end of thebias member 320. A surface of the wall 312 provides another retainingportion at another end of the bias member such that the bias member iscaptured between the terminating end 322 and the wall 312.

The second cavity 310 is further defined by a first surface 324 of thering plunger 316 which interfaces with a second surface of the wall 312to define the second cavity 310. The second cavity 310 includes avariable volume, the volume of which is adjustable as a function ofmovement of the ring plunger 316 along a direction 326, in eitherdirection. A body portion 328 of the plunger 316 defines the firstsurface 324 and a second surface 330. The second surface 330 defineswith adjacent walls of the case 304 a third cavity 332, which has agenerally ring shaped volume extending around the body portion 328. Thethird cavity 332 is coupled to an aperture 334 defined by the case 304which is configured to receive a fitting 336. The fitting 336 receives apilot pressure as described above with regard to FIGS. 3 and 4.

In this embodiment, however, the application of a pilot pressure at theaperture 334 moves the ring plunger 316 away from the wall 312 andtoward a pump housing 338, which provides a sidewall for the housing andwhich is fixedly attached to the case 304. The pump housing 338 includesa central channel 340 configured to support a gear 342. Upon applicationof the pilot pressure at the aperture 334, the body portion 328 movestoward the pump housing 338 to provide a working fluid pressure to thebias element 342. In the illustrated state, the application of the pilotpressure collapses the spring 320. Consequently, to maintain the gear342 in a working state, the pilot pressure is continuously maintained.

If, however, it is desired to prevent the gear 342 from pumping, thepilot pressure is removed and the spring force of the bias element 320moves the body portion 328 away from the gear 342 and toward the cover306. Fluid pressure is reduced and while the gear 342 may still turn,the fluid pressure is reduced sufficiently to reduce fluid flow beingdisplaced by the pump.

In this embodiment, should the solenoid providing the pilot pressurefail to operate, the cavity 310 collapses and sufficient pressure is notavailable to enable the gear 342 to turn as needed. Consequently, asdescribed herein, each of the different embodiments provides a differentstate of the pump when the pilot pressure is unavailable. The describedembodiments, therefore, provide a relatively inexpensive, but extremelyeffective unloading mechanism for gear pumps when no flow and low energystates are desired, which saves one or both of power and fuel.

While exemplary embodiments incorporating the principles of the presentinvention have been disclosed hereinabove, the present invention is notlimited to the disclosed embodiments. Instead, this application isintended to cover any variations, uses, or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of the appended claims.

1. A fluid transfer pump comprising: a housing defining a cavity and apassage having an inlet disposed at an external portion of the housing;a movable member disposed within the cavity, the movable member defininga first chamber, wherein a volume of the first chamber is adjustablewith movement of the movable member; a gear set disposed at the firstchamber, the gear set including a first gear and a second gear, whereinthe first chamber is defined by ends of the first gear, the second gear,and the movable member, the first chamber being configured to hold afirst volume of fluid in fluid communication with the ends of the firstgear and the second gear; and a bias element disposed at the movablemember, wherein the bias element is configured to adjust the location ofthe movable member to a first position defining the first volume of thefirst chamber and to a second position defining a second volume of thefirst chamber; wherein the movable member partially defines a secondchamber in which a volume of the second chamber is adjustable withmovement of the movable member, and the passage is configured to directa pilot fluid from the inlet to the second chamber to adjust a size ofthe second chamber from a first size to a second size; further wherein,a first fluid pressure in the first chamber is greater than a secondfluid pressure in the first chamber and the second fluid pressure isinsufficient to provide a working pressure.
 2. The fluid transfer pumpof claim 1, wherein the housing includes a first cover disposed at aside of the housing and the bias element is disposed between the movablemember and the first cover of the housing.
 3. The fluid transfer pump ofclaim 1 further comprising a seal disposed between the first chamber andthe second chamber, wherein the seal isolates fluid in the first chamberfrom fluid in the second chamber.
 4. The fluid transfer pump of claim 1wherein the housing includes an interior wall and the bias element isdisposed between the interior wall and the moveable member.
 5. A fluidtransfer pump comprising: a housing defining a first cavity and apassage operatively coupled to a second cavity, the passage configuredto provide fluid transfer into the second cavity, wherein the housingfurther includes an interior wall disposed in the housing and a fixedside wall disposed at an end of the housing; a movable member movablydisposed within the housing; a bias element disposed between the fixedside wall and the movable member, wherein a fluid applied to the passageinto the second cavity changes the bias force of the bias element andthe location of the movable member with respect to the interior wall tochange a volume of the first cavity; a gear disposed at the firstcavity, wherein the volume of the first cavity is configured to providea fluid pressure within the first cavity to enable the fluid transferpump to transfer fluid at a working pressure; a spindle coupled to thegear, wherein the gear partially defines the interior wall, and thespindle extends through both the gear and the movable member; and afixed second sidewall, a second sidewall bearing located at the fixedsecond sidewall, and movable member bearing located at the movablemember; wherein the movable member defines with the housing the firstcavity and the second cavity, wherein an increase in volume of thesecond cavity increases the volume of the first cavity; further wherein,the spindle extends from the second sidewall bearing to the movablemember bearing.
 6. The fluid transfer pump of claim 5, furthercomprising a seal disposed between the housing and the movable member,wherein the seal substantially prevents fluid located in the secondcavity from moving to the first cavity.
 7. The fluid transfer pump ofclaim 6 wherein the bias element is configured to move the movablemember toward the gear, and the application of a fluid to the secondcavity is configured to move the movable member away from the gear andto compress the bias element, thereby increasing the size of the firstcavity.
 8. A fluid transfer pump comprising: a housing defining a firstcavity and a passage operatively coupled to a second cavity, the passageconfigured to provide fluid transfer into the second cavity, wherein thehousing further includes a fixed side wall disposed at an end of thehousing; a movable member movably disposed within the housing; a biaselement disposed between the fixed side wall and the movable member,wherein a fluid applied to the passage into the second cavity compressesthe bias element and changes the location of the movable member withrespect to the fixed side wall to change a volume of the first cavity; agear disposed at the first cavity and coupled to a spindle; and a fixedsecond sidewall, a second sidewall bearing located at the fixed secondsidewall, and a movable member bearing located at the movable member;wherein the movable member defines with the housing the first cavity andthe second cavity, wherein an increase in volume of the second cavityincreases the volume of the first cavity; further wherein, the spindleextends from the second sidewall bearing to the movable member bearing.9. The fluid transfer pump of claim 8, further comprising a sealdisposed between the housing and the movable member, wherein the sealsubstantially prevents fluid located in the second cavity from moving tothe first cavity.
 10. The fluid transfer pump of claim 8, wherein thebias element applies a biasing force to the movable member toward thegear, and the application of a fluid at a pilot pressure to the secondcavity moves the movable member away from the gear and compresses thebias element, thereby increasing the size of the first cavity.
 11. Afluid transfer pump comprising: a housing including a casing and an endcover connected to the casing; a gear set disposed in the housing; anend plate movable with respect to the housing and disposed between thegear set and the end cover, the end plate movable between a firstposition and second position, wherein in the first position the endplate defines a space between the end plate and the end cover and in thesecond position the end plate defines a first cavity between the gearset and the end plate, wherein the end plate is sealed with the gear setto define the first cavity; and an aperture disposed in the housing andfluidly coupled to the end plate, wherein pilot fluid provided at theaperture moves the end plate between the first position and the secondposition to provide a working pressure for operation of the fluidtransfer pump.
 12. The fluid transfer pump of claim 11 furthercomprising a bias element disposed at the end plate, wherein the biaselement is configured to adjust the location of the end plate withrespect to the end cover.
 13. The fluid transfer pump of claim 12wherein the bias element includes a first bias element and a second biaselement, and wherein the first bias element and the second bias elementforce the end plate away from the end cover in the absence of pilotfluid provided at the aperture.
 14. The fluid transfer pump of claim 13wherein the end plate includes a first aperture and the first biaselement is located in the first aperture.
 15. The fluid transfer pump ofclaim 14 wherein the end plate includes a second aperture and the secondbias element is located in the second aperture.
 16. The fluid transferpump of claim 12 wherein the gear set includes a first gear and a secondgear, and further comprising a first shaft extending through the casing,the first gear, and the end plate.
 17. The fluid transfer pump of claim16 further comprising a second shaft extending through the second gearand the bias element, wherein the second shaft extends no further thanthe casing and no further than the end plate.
 18. The fluid transferpump of claim 17 further comprising a plurality of bearings, wherein thefirst shaft and the second shaft are respectively sealed for rotation atthe casing and at the end plate with the plurality of bearings.
 19. Thefluid transfer pump of claim 12 wherein an interface between the endplate and the end cover defines a second cavity coupled to the aperture,wherein a pilot fluid applied to the aperture adjusts a volume of thesecond cavity.
 20. The fluid transfer pump of claim 19 furthercomprising a plurality of seals each of which is located at theinterface between the end plate and the end cover.